The present invention relates to synchronous generators, and in particular to active damping of synchronous generators.
Synchronous generators are used in a variety of applications to convert mechanical energy provided by an engine to alternating current (AC) electrical energy. In particular, synchronous generators are used in applications such as aboard aircraft to generate the AC electrical energy necessary to support on-board electrical systems.
In a typical topology, an engine generates mechanical energy that is provided through a gearbox to a synchronous generator or to multiple synchronous generators. A shaft transmits the mechanical energy from the gearbox to the synchronous generator. Due to a multitude of competing mechanical design considerations, the shaft may be relatively long and mechanically compliant. The inertias associated with the engine, the gearbox, the synchronous generator, and other gearbox driven accessories in combination with the mechanical compliance or spring rates of the mechanical drivetrain, including the generator shaft, create a distributed mechanical spring-mass system that has associated torsional resonances. There are multiple torsional modes and associated resonances that involve the generators for multiple direct-driven generators on a common gearbox. Engine gearboxes typically exhibit very lightly damped characteristics, and because the synchronous generator is controlled to maintain an AC voltage, it presents a near constant power load characteristic to the mechanical drivetrain that results in negative damping for disturbance frequencies that are within the generator's voltage regulation bandwidth. In certain situations, depending on the generator speed, the generator electrical load, and the net effective damping in the overall mechanical drivetrain, the torsional resonance of the spring-mass system involving the generator or generators can lead to large, undesirable torsional oscillations and mechanical failures in the system. Mechanical damping may be used to offset the negative damping characteristic of the synchronous generator or generators and thus dampen the torsional oscillations in the spring-mass system, but mechanical damping requires additional parts that increase the weight and cost of the system.